The field of the invention is magnetic resonance angiography ("MRA"), and particularly, dynamic studies of the human vasculature using contrast agents which enhance the NMR signals.
Diagnostic studies of the human vasculature have many medical applications. X-ray imaging methods such as digital subtraction angiography ("DSA") have found wide use in the visualization of the cardiovascular system, including the heart and associated blood vessels. Images showing the circulation of blood in the arteries and veins of the kidneys and the carotid arteries and veins of the neck and head have immense diagnostic utility. Unfortunately, however, these x-ray methods subject the patient to potentially harmful ionizing radiation and often require the use of an invasive catheter to inject a contrast agent into the vasculature to be imaged.
One of the advantages of these x-ray techniques is that image data can be acquired at a high rate (i.e. high temporal resolution) so that a sequence of images may be acquired during injection of the contrast agent. Such "dynamic studies" enable one to select the image in which the bolus of contrast agent is flowing through the vasculature of interest. Earlier images in the sequence may not have sufficient contrast in the suspect vasculature, and later images may become difficult to interpret as the contrast agent reaches veins and diffuses into surrounding tissues. Subtractive methods such as that disclosed in U.S. Pat. No. 4,204,225 entitled "Real-Time Digital X-ray Subtraction Imaging" may be used to significantly enhance the diagnostic usefulness of such images.
Magnetic resonance angiography (MRA) uses the nuclear magnetic resonance (NMR) phenomenon to produce images of the human vasculature. When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins, and after the excitation signal B.sub.1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. Each measurement is referred to in the art as a "view" and the number of views determines the resolution of the image. The resulting set of received NMR signals, or views, are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. The total scan time is determined in part by the number of measurement cycles, or views, that are acquired for an image, and therefore, scan time can be reduced at the expense of image resolution by reducing the number of acquired views.
MR angiography (MRA) has been an active area of research. Two basic techniques have been proposed and evaluated. The first class, time-of-flight (TOF) techniques, consists of methods which use the motion of the blood relative to the surrounding tissue. The most common approach is to exploit the differences in signal saturation that exist between flowing blood and stationary tissue. This is know as flow-related enhancement, but this effect is misnamed because the improvement in blood-tissue contrast is actually due to the stationary tissues experiencing many excitation pulses and becoming saturated. Flowing blood, which is moving through the excited section, is continually refreshed by spins experiencing fewer excitation pulses and is, therefore, less saturated. The result is the desired image contrast between the high-signal blood and the low-signal stationary tissues.
MR methods have also been developed that encode motion into the phase of the acquired signal as disclosed in U.S. Pat. No. Re. 32,701. These form the second class of MRA techniques and are known as phase contrast (PC) methods. Currently, most PC MRA techniques acquire two images, with each image having a different sensitivity to the same velocity component. Angiographic images are then obtained by forming either the phase difference or complex difference between the pair of velocity-encoded images. Phase contrast MRA techniques have been extended so that they are sensitive to velocity components in all three orthogonal directions.
Despite the tremendous strides made in recent years, at many clinical sites MRA is still considered a research tool and is not routinely used in clinical practice. More widespread application of either TOF or PC techniques is hampered by the presence of a variety of deleterious image artifacts, which can mask and, in some cases, even mimic pathology. These artifacts generally result in a lower specificity, as well as compromised sensitivity.
To enhance the diagnostic capability of MRA a contrast agent such as gadolinium can be injected into the patient prior to the MRA scan. As described in U.S. Pat. No. 5,417,213 the trick is to acquire the central k-space views at the moment the bolus of contrast agent is flowing through the vasculature of interest. This is not an easy timing to achieve as part of a routine clinical procedure and a number of solutions have been proposed.
In U.S. Pat. No. 5,713,358 entitled "Method For Producing A Time-Resolved Series Of 3D Magnetic Resonance Angiograms During The First Passage Of Contrast Agent" a method is described for rapidly acquiring a series of 3D MRA data sets during a dynamic study. The objective is to acquire as many images as possible during the dynamic study so that one of the images will depict the subject when the vasculature is at maximum contrast. This requires that fast NMR data acquisition methods be used.
Most NMR scans currently used to produce medical images require many minutes to acquire the necessary data for a clinically useful image. The reduction of this scan time to seconds rather than minutes is the major obstacle in performing clinical dynamic studies using MRI methods. The most common MRI method currently used for non-triggered, time-resolved imaging is to use an echo-planar imaging ("EPI") pulse sequence such as that first described by Peter Mansfield (J. Phys. C. 10: L55-L58, 1977). In principle the EPI scan enables imaging of dynamic processes occurring with periods measured on the order of a few hundred milliseconds. However, time-resolved EPI has been considered un-suitable for contrast enhanced MRA because it exhibits a low contrast between blood and surrounding tissues due to the long time intervals (e.g. 100 ms) between RF excitations. EPI also has enhanced sensitivity to a variety of flow-related artifacts, and EPI images can be blurred due to T.sub.2.sup.* -modulation of k-space.
A number of methods have been developed to increase the temporal resolution of MRI scans using pulse sequences that are applicable to MRA. In a method known in the art as "MR fluoroscopy" and described in U.S. Pat. No. 4,830,012, the subject is scanned by continuously and repeatedly acquiring the N phase encoding views needed for a complete image. Rather than waiting for an entirely new set of N views before reconstructing the next image, however, images are reconstructed at a much higher rate by using the most recent N views. In other words, an image is reconstructed from newly acquired views as well as views used in reconstructing previous images in the dynamic study. While very high temporal rates are achieved with MR fluoroscopy, the image contrast is not satisfactory for MRA because the central views in k-space, which dominate the overall image contrast, are still updated at the much slower inherent scan rate (i.e. NxTR).
Another method for increasing temporal resolution of MRI images is referred to in the art as "keyhole" imaging. As described, for example, by R.A. Jones, et al. in "Dynamic, Contrast Enhanced, NMR Perfusion Imaging Of Regional Cerebral Ischaemia In Rats Using K-Space Substitution", SMR Eleventh Annual Meeting 1992 abs. 1138, a sequence of images is acquired during a dynamic study in which a contrast agent is injected in the subject. The first image in the sequence is a reference image in which all the phase encoding views (e.g. 128 views) are acquired. Subsequent images are produced, however, by only acquiring the central views (e.g. the central 32 views). These keyhole scans can obviously be acquired much faster than complete scans and the temporal rate is increased proportionately. The keyhole images are reconstructed using the most recent central k-space views combined with the outer, peripheral k-space views from the reference scan. Unfortunately, in situations where the low spatial frequency changes in the reconstructed images do not capture the evolution of the dynamic study, k-space keyhole imaging is not appropriate. This is a problem when contrast changes in small regions are to be studied, and in such studies the number of central views acquired must be increased to the point where the gain in temporal resolution is lost.
Related to the k-space keyhole imaging method is a method known in the art as limited field of view ("FOV") dynamic imaging. As described, for example, by Hu and Parrish, published in Magnetic Resonance in Medicine, Vol. 31, pp. 691-694, 1994, and by Frederickson and Pelc, 3rd SMR, 1, 197.1995; this method is applied to dynamic studies in which the changing part of the image occupies no more than one half the full FOV. A reference image representing the static part of the image is produced at the beginning of the study and a series of images encompassing only the dynamic, central portion of the image are produced using half the number of phase encoding views. These dynamic images can be acquired at a higher temporal rate because only half the number of views (either the odd or even views) need be acquired. The dynamic and static portions of the image are combined to produce a corresponding series of full FOV images. Of course, if changes occur in the static portion of the image, the information obtained from these regions will no longer accurately remove artifacts aliased into the small FOV.
Dynamic MRA studies currently use fast gradient recalled echo pulse sequences because their short repetition times (TR) enable a maximum number of views to be acquired at a given temporal frame rate. As indicated above by the many schemes which have been proposed, there is a strong need for methods which will enable the quality of the acquired images to be improved without slowing the temporal frame rate.